1. Field of the Invention
The present invention pertains to active sensor devices such as motion, edge, frequency and speed sensors or counting devices. In particular, it relates to gear tooth sensors which sense the presence or absence of gear teeth as they rotate past a sensor device, and especially the invention relates to such sensors which use a magnet and a Hall cell integrated circuit and which do not require a specific orientation between the sensor and the gear.
2. Description of the Prior Art
Prior art approaches to gear tooth sensors have included magneto-resistive devices, a proximity switch and other similar devices. The problems with the previous devices have been cost, temperature range, manufacturing tolerances and air gaps.
As a preliminary note, the basic Hall sensor is simply a small sheet of semiconductor material. A constant voltage source forces a constant bias current to flow in the longitudinal direction in the semiconductor sheet. The output, a voltage measured across the width of the sheet, reads near zero if a magnetic field is not present. If the biased Hall sensor is placed in a magnetic field oriented transversely to the Hall current, the voltage output is in direct proportion to the strength of the magnetic flux component at right angles to the hall cell. The basic Hall sensor is essentially a transducer that will respond with an output voltage if the applied magnetic field changes in any manner.
Principles of operation are discussed in the "Sprague Integrated Circuits Data Book", WR-503, in Section 9, published by Sprague Electric Company, North Adams, Mass. Hall cells can be operated by a unidirectional field where the parameter B (flux density) expressed in Gauss can be modulated to produce a level of flux either below or above a threshold level which is established by either a bias flux level or by an electronic bias in the circuit reading the Hall cell. This type of component is termed a Hall digital switch, and is typified by Sprague UGN 3040T described on page 9-11 of the above-cited Sprague Data Book.
Hall cells can also be operated by a bidirectional field wherein the parameter B (flux density) expressed in Gauss can be alternated in polarity to produce flux passing through the cell first in one direction and then in the other direction. This type of component is termed a Hall digital latch and is typified by UGN 3075T on page 9-15 of the above-cited Sprague Data Book.
The Sprague Data Book also illustrates a technique for detecting what could be termed "gear teeth", but this technique has the disadvantage that the electronics are A-C coupled and therefore cannot be used where direct coupling is required.
Many attempts to make a gear tooth sensor which is direct coupled and which uses one or two stationary magnets to produce flux have been made. Such attempts usually consist of techniques employing the previously mentioned digital switch type components in one form or another wherein a passing gear tooth modulates the magnitude of flux about a fixed bias B. These techniques are very susceptible to changes in temperature and changes in the distance between the gear tooth and the sensor, commonly known as the air gap.
The prior art does not show a method or device for bending or steering the flux paths using a single magnet with a gear to cause a Hall switch or latch to change levels as the gear passes. The prior art used a vane for flux shunting or moving magnets. For example, most automotive ignition systems interrupt a magnetic field with a vane.
The cross-reference application discloses a technique using direct coupled Hall devices for sensing gear teeth or other discontinuities of iron or similar ferrous magnetic or magnetic targets in commercially feasible embodiments which can be constructed using presently available electronic components.